I've always found this passage a bit confusing.

I get the first three.  The Father (God), the Word (Jesus), and the Holy Spirit (the Holy Spirit).  But what about three?  The Spirit, the Water, and the Blood.

1 John 5:6-8(NKJV)

This is He who came by water and blood—Jesus Christ; not only by water, but by water and blood. And it is the Spirit who bears witness, because the Spirit is truth. 

For there are three that bear witness in heaven: the Father, the Word, and the Holy Spirit; and these three are one. 

And there are three that bear witness on earth: the Spirit, the water, and the blood; and these three agree as one.